Russian Foreign Minister Sergei Lavrov has said the Ukrainian government must stop its military operation and agree a ceasefire with separatists in the east of the country.

Russia's Foreign Ministry said Lavrov made the remark in a telephone call with U.S. Secretary of State John Kerry on June 11.

Meanwhile, Ukraine's new president has signaled he would be ready to hold talks with opponents in eastern Ukraine if pro-Russian separatists waging an insurgency there agreed to lay down their weapons.

Petro Poroshenko was quoted by his press office as telling the governor of the Donetsk region of east Ukraine that he would not rule out holding "roundtable" talks with "different parties."

Poroshenko also spoke by phone with U.S. Vice President Joe Biden who said Moscow must recognize Poroshenko as Ukraine's legitimate leader, stop supporting pro-Russian separatists and prevent arms from flowing over the border.